The beneifts provided from sowing perennial pastures need to be balanced against problems that may arise from their widespread use. One potential problem is the development of a continuous herbaceous ‘green bridge’ for legume and cereal pests and pathogens that would otherwise be unable to persist over summer.
Such a ‘green bridge’ is absent with annual pastures, which senesce in late spring and only regenerate following the autumn rains. Also, pathogens and pests which are able to survive over summer without a ‘green bridge’, e.g. inside dormant seed, in trash or those in diapause, could continue to build-up in perennial pastures.
With annual pastures, epidemics of such pathogens and outbreaks of such pests cease at the end of the growing season and then start again from a low base when the pasture regenerates in autumn.
In WA, the seasonally dry conditions, which can extend from late spring through to late autumn, ensure a substantial, continuous bridge of green foliage is rarely present away from isolated ‘wet’ spots.
Even in years with good out-of-season rainfall the perennial pastures dry off and/or drop leaves in response to moisture stress. The summer dry period extends for four to seven months in any year. While this reduces leaf diseases, which only survive on green foliage (e.g. rust), it has less impact on viruses, which can persist in the root system of plants.
Another issue is that many perennial legume pastures are being sown with untested seed from regions where a number of seed-borne diseases of legumes occur. The widespread sowing of untested seed risks the potential large-scale introduction of seed-borne diseases.
A survey of commercial lucerne seed entering WA from the eastern States showed 80% of the samples were infected with AMV (levels of 0.1-4%) and 11% were infected with CMV (0.1-0.3%), although no CMV was detected in survey samples taken at random in the field.
Diseases reduce herbage production and seed yields of legume and grass pastures, reducing their persistence and favouring the build-up of weeds. In addition, diseases spread from legume pastures to nearby grain legume crops and annual legume pastures, or from perennial grass pastures to cereals.
If straightforward control measures are implemented, the disease risks from growing perennial pastures can be reduced.
Virus diseases:
In WA, detailed surveys of virus occurrence have been undertaken in perennial pastures of lucerne, irrigated white clover, perennial ryegrass and kikuyu, with less detailed studies on irrigated perennial grass pastures containing couch grass.
These surveys revealed widespread infection with AMV in lucerne and white clover, WCMV in white clover, RyMV in perennial ryegrass and BYDV and CYDV in various perennial grasses. Seed-borne infection with AMV was common in commercial lucerne seed stocks.
Symptoms caused by viruses in pasture species vary in intensity and type depending on the combination of virus and plant species infected. Mottle, pallor, vein clearing, reduced size and deformation are common leaf symptoms, while dwarfing is a common whole plant symptom.
Factors that favour virus build-up and the risk of epidemics and increased production losses include long-term perennial pastures, the removal of non-host pasture species, e.g. grasses from legume pastures and legumes from grass pastures and a range of other factors that depend on the mode of transmission of the virus.
Control measures for viruses:
The aim with control measures is to minimise the build-up of virus infection and consequent losses to reduce the potential for the perennial pasture to be a source of infection to adjacent annual crop and pasture paddocks.
The potential damage from infecting crops is sometimes greater than the direct damage to the pasture. The incidence of virus infection increases with the age of a stand. Economic loss is likely when incidences are high.
With viruses that are not seed-borne, rotation with a crop after about three years will reduce disease build-up, but this is only possible with phase farming. Other control measures vary with the mode of transmission of the virus.
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